HashMap 和ConcurrentHashMap

   static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16  默认初始值

 
    static final int MAXIMUM_CAPACITY = 1 << 30;   //最大值

    static final float DEFAULT_LOAD_FACTOR = 0.75f;  //加载因子

    static final int TREEIFY_THRESHOLD = 8;      //转化为红黑树的链表的最小长度

    static final int MIN_TREEIFY_CAPACITY = 64;   //转化为红黑树的数组的最小长度

    static final int UNTREEIFY_THRESHOLD = 6;    //退化为链表的长度

    transient Node[] table;  //存放的数组

    transient Set> entrySet;  //键值对的集合

    transient int size;  //元素个数

    transient int modCount;  //并发修改异常

    final float loadFactor;   //自定义加载因子

   hash(key)  返回object的hashCode的值
   helpTransfer(Node[] tab, Node f)  如果正在扩容，该线程也加入到扩容中

  public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {

        Node[] tab; //存放元素的数组
        Node p;  //产生hash冲突时的首节点
        int n, i;  //我们首先可能会想到采用%取余的操作来实现。但是，重点来了：“取余(%)操 作中如果除数是2的幂次则等价于与其除数减一的与(&)操作（也就是说 hash%length==hash&(length-1)的前提是 length 是2的 n 次方；）。” 并且 采用二进制位操作 &，相对 于%能够提高运算效率，这就解释了 HashMap 的长度为什么是2的幂次方
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;                    //1.如果没有初始化就进行初始化
        if ((p = tab[i = (n - 1) & hash]) == null)         //2.通过hash值计算数组的索引，如果为空，则直接创建节点插入
            tab[i] = newNode(hash, key, value, null);
        else {
//3.不为空，产生了hash冲突
            Node e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;    //4.key相等或key是同一个引用，e记录下来
            else if (p instanceof TreeNode)    //5.如果当前是红黑树，转化成红黑树节点的插入
                e = ((TreeNode)p).putTreeval(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {   //6.如果是链表的话，对链表进行遍历
                    if ((e = p.next) == null) {          //7.到达链表末尾，进行插入，如果链表长度大于等于8时，再在(treeifyBin)里面判断数组长度是否大于等于64，如果时则转化为红黑树，否则直接扩容。（此时e为空了）
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                      //8.如果存在覆盖，也跳出循环
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }

         //9.e为空就是红黑树的插入，e不为空就是链表的覆盖，覆盖之后size不变，直接返回。
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }

       //10，插入新节点之后，modcount自己，防止并发修改异常，判断是否需要扩容，如果是则进行扩容。
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

final Node[] resize() {
        Node[] oldTab = table;        //保存老数组的值
        int oldCap = (oldTab == null) ? 0 : oldTab.length; // 老数组长度
        int oldThr = threshold;                 //老阔容得临界值
        int newCap, newThr = 0;//新容量，新临界值
        if (oldCap > 0) {
            if (oldCap >= MAXIMUM_CAPACITY) {   //1.如果数组长度达到Integer的最大值，就不进行扩容了
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&   //2。否则就扩到原来的2倍
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold
        }
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;     //3.初始化
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }

          //3.重新计算临界值
        if (newThr == 0) {
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})

      //4.将老数组得元素迁移到新数组中
        Node[] newTab = (Node[])new Node[newCap];
        table = newTab;   //将table指向新数组
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {
                Node e;
                if ((e = oldTab[j]) != null) {   //5.数组不为空得需要迁移
                    oldTab[j] = null;
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;  //6.只有一个元素得链表得迁移
                    else if (e instanceof TreeNode)
                        ((TreeNode)e).split(this, newTab, j, oldCap);  //7.是红黑树时得迁移
                    else { // preserve order
                        Node loHead = null, loTail = null;
                        Node hiHead = null, hiTail = null;
                        Node next;
                        do {
                            next = e.next;  //8.从链表头开始迁移
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;        //   9.尾插法
                            }
                            else {
                                if (hiTail == null)
                                    hiHead = e;
                                else
                                    hiTail.next = e;
                                hiTail = e;
                            }
                        } while ((e = next) != null);
                        if (loTail != null) {  //索引不变
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {  //索引变为原来得2倍
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }

ConcurrentHashMap

    //全部用volatile修饰
   transient volatile Node[] table;

    private transient volatile Node[] nextTable; //扩容用的数组

    private transient volatile long baseCount;

    private transient volatile int sizeCtl;

    private transient volatile int transferIndex;

    private transient volatile int cellsBusy;

    private transient volatile CounterCell[] counterCells;

 static final int NCPU = Runtime.getRuntime().availableProcessors();//扩容时采用多线程扩容

 private static final int MIN_TRANSFER_STRIDE = 16;//扩容时的每个转移步骤的最小重新局数，细分范围以允许多个调整器线程。此值用作下限，以避免大小调整器遇到过多内存争用。该值应至少为默认容量。，细分范围

private static final int RESIZE_STAMP_BITS = 16;//时间戳，防止cas多线程扩容时引发的ABA问题

resizeStamp(n)；//返回时间戳

    //计算索引下标的第一个值
static final  Node tabAt(Node[] tab, int i) {
        return (Node)U.getReferenceAcquire(tab, ((long)i << ASHIFT) + Abase);
    }

//cas插入节点
    static final  boolean casTabAt(Node[] tab, int i,
                                        Node c, Node v) {
        return U.compareAndSetReference(tab, ((long)i << ASHIFT) + Abase, c, v);
    }
//cas扩容
    static final  void setTabAt(Node[] tab, int i, Node v) {
        U.putReferenceRelease(tab, ((long)i << ASHIFT) + Abase, v);
    }

put

spread(int h) {  //二次哈希
        return (h ^ (h >>> 16)) & HASH_BITS;
    }

 public V put(K key, V value) {
        return putVal(key, value, false);
    }

    
    final V putVal(K key, V value, boolean onlyIfAbsent) {
//1.键和值都不能为空
        if (key == null || value == null) throw new NullPointerException();
//2.二次hash键得hashCode
        int hash = spread(key.hashCode());
        int binCount = 0;
        for (Node[] tab = table;;) {
            Node f; 
            int n, i, fh;
            K fk; V fv;
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();   //3.没有初始化就进行初始化
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {  //4.计算索引为空时，cas进行插入
                if (casTabAt(tab, i, null, new Node(hash, key, value)))
                    break;                   // no lock when adding to empty bin
            }
            else if ((fh = f.hash) == MOVED)    //5.如果此时正在扩容，就把节点传过去
                tab = helpTransfer(tab, f);
            else if (onlyIfAbsent // check first node without acquiring lock//6.如果头节点得引用和key相同，则退出
                     && fh == hash
                     && ((fk = f.key) == key || (fk != null && key.equals(fk)))
                     && (fv = f.val) != null)
                return fv;       
            else {
                V oldVal = null;
                synchronized (f) {  //7.否中使用synchronized进行插入，只锁住头节点，并发度为数组得长度size。
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {
                            binCount = 1;
                            for (Node e = f;; ++binCount) {
                                K ek;
                                if (e.hash == hash &&
                                    ((ek = e.key) == key ||
                                     (ek != null && key.equals(ek)))) {  //8.相等则覆盖
                                    oldVal = e.val;
                                    if (!onlyIfAbsent)
                                        e.val = value;
                                    break;
                                }
                                Node pred = e;
                                if ((e = e.next) == null) {   //9.否中插入
                                    pred.next = new Node(hash, key, value);
                                    break;
                                }
                            }
                        }
                        else if (f instanceof TreeBin) {   //10.如果是红黑树，则进行红黑树的插入
                            Node p;
                            binCount = 2;
                            if ((p = ((TreeBin)f).putTreeval(hash, key,
                                                           value)) != null) {
                                oldVal = p.val;
                                if (!onlyIfAbsent)
                                    p.val = value;
                            }
                        }
                        else if (f instanceof ReservationNode)
                            throw new IllegalStateException("Recursive update");
                    }
                }

                if (binCount != 0) { //11.进行了插入判断是否需要扩容
                    if (binCount >= TREEIFY_THRESHOLD)
                        treeifyBin(tab, i);
                    if (oldVal != null)
                        return oldVal;
                    break;
                }
            }
        }
        addCount(1L, binCount);//12.判断是否需要扩容
        return null;
    }

扩容函数

private final void transfer(Node[] tab, Node[] nextTab) {
        int n = tab.length, stride;
//1.计算细分范围
        if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
            stride = MIN_TRANSFER_STRIDE; // subdivide range
        if (nextTab == null) {            // initiating
            try {
                @SuppressWarnings("unchecked")
                Node[] nt = (Node[])new Node[n << 1];
                nextTab = nt;
            } catch (Throwable ex) {      // try to cope with OOME
                sizeCtl = Integer.MAX_VALUE;
                return;
            }
            nextTable = nextTab;
            transferIndex = n;
        }
        int nextn = nextTab.length;
        ForwardingNode fwd = new ForwardingNode(nextTab);
        boolean advance = true;
        boolean finishing = false; // to ensure sweep before committing nextTab
        for (int i = 0, bound = 0;;) {  //2.从第一个桶开始扩容
            Node f;
            int fh;
            while (advance) {
                int nextIndex, nextBound;
                if (--i >= bound || finishing)
                    advance = false;
                else if ((nextIndex = transferIndex) <= 0) {
                    i = -1;
                    advance = false;
                }
                else if (U.compareAndSetInt
                         (this, TRANSFERINDEX, nextIndex,
                          nextBound = (nextIndex > stride ?
                                       nextIndex - stride : 0))) {
                    bound = nextBound;
                    i = nextIndex - 1;
                    advance = false;
                }
            }
            if (i < 0 || i >= n || i + n >= nextn) {
                int sc;
                if (finishing) {   //扩容完成table指向新的数组
                    nextTable = null;
                    table = nextTab;
                    sizeCtl = (n << 1) - (n >>> 1);
                    return;
                }
                if (U.compareAndSetInt(this, SIZECTL, sc = sizeCtl, sc - 1)) {
                    if ((sc - 2) != resizeStamp(n) << RESIZE_STAMP_SHIFT)
                        return;
                    finishing = advance = true;
                    i = n; // recheck before commit
                }
            }
            else if ((f = tabAt(tab, i)) == null)  //只有一个元素的桶，cas直接插入
                advance = casTabAt(tab, i, null, fwd);
            else if ((fh = f.hash) == MOVED)
                advance = true; // already processed
            else {
                synchronized (f) {     //获取第i个桶，cas插入新的table中
                    if (tabAt(tab, i) == f) {
                        Node ln, hn;
                        if (fh >= 0) {
                            int runBit = fh & n;
                            Node lastRun = f;
                            for (Node p = f.next; p != null; p = p.next) {
                                int b = p.hash & n;
                                if (b != runBit) {
                                    runBit = b;
                                    lastRun = p;
                                }
                            }
                            if (runBit == 0) {
                                ln = lastRun;
                                hn = null;
                            }
                            else {
                                hn = lastRun;
                                ln = null;
                            }
                            for (Node p = f; p != lastRun; p = p.next) {
                                int ph = p.hash; K pk = p.key; V pv = p.val;
                                if ((ph & n) == 0)
                                    ln = new Node(ph, pk, pv, ln);
                                else
                                    hn = new Node(ph, pk, pv, hn);
                            }
                            setTabAt(nextTab, i, ln);
                            setTabAt(nextTab, i + n, hn);
                            setTabAt(tab, i, fwd);
                            advance = true;
                        }
                        else if (f instanceof TreeBin) {
                            TreeBin t = (TreeBin)f;
                            TreeNode lo = null, loTail = null;
                            TreeNode hi = null, hiTail = null;
                            int lc = 0, hc = 0;
                            for (Node e = t.first; e != null; e = e.next) {
                                int h = e.hash;
                                TreeNode p = new TreeNode
                                    (h, e.key, e.val, null, null);
                                if ((h & n) == 0) {
                                    if ((p.prev = loTail) == null)
                                        lo = p;
                                    else
                                        loTail.next = p;
                                    loTail = p;
                                    ++lc;
                                }
                                else {
                                    if ((p.prev = hiTail) == null)
                                        hi = p;
                                    else
                                        hiTail.next = p;
                                    hiTail = p;
                                    ++hc;
                                }
                            }
                            ln = (lc <= UNTREEIFY_THRESHOLD) ? untreeify(lo) :
                                (hc != 0) ? new TreeBin(lo) : t;
                            hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) :
                                (lc != 0) ? new TreeBin(hi) : t;
                            setTabAt(nextTab, i, ln);
                            setTabAt(nextTab, i + n, hn);
                            setTabAt(tab, i, fwd);
                            advance = true;
                        }
                        else if (f instanceof ReservationNode)
                            throw new IllegalStateException("Recursive update");
                    }
                }
            }
        }
    }